1. Field of the Invention
The invention relates to an apparatus for the accurately positioned tensioning of a printing plate on the printing cylinder of a printing mechanism, with the circumference of the printing cylinder being provided with a groove that extends in the axial direction of the cylinder so as to accommodate tensioning and fastening means for the printing plate. The tensioning and fastening means include a front tensioning bar for the start of printing of the printing plate and a rear tensioning bar for the end of printing of the printing plate, with the rear tensioning bar being disposed ahead of the front tensioning bar in the direction of rotation of the printing cylinder. Means are provided for the releasable fastening of the ends of the printing plate to the two tensioning bars and means for displacing the rear tensioning bar in the circumferential direction of the printing cylinder over a longer tensioning path for the purpose of tensioning the printing plate over the circumference of the cylinder and means for displacing the front tensioning bar in the circumferential direction of the printing cylinder over a smaller adjustment path for adjusting the image to be printed in the press. The adjustment means for adjusting the printing plate cooperate with a stop fixed to the cylinder as the zero position.
2. Background Information
Such devices are known in many embodiments. As an example, reference can be made to DE-OS (German Laid-Open Patent Application) 3,843,395. This prior art has in common that the stop defining the zero position of the front tensioning bar is the front groove wall. A plurality of tensioning screws are screwed into the front tensioning bar, spaced from one another and distributed over its length. These screws must be set manually so that the zero position of the printing cylinder comes into the desired position as required for the total press run. However, this manner of setting the zero position poses problems. It is not possible to take the front groove wall as such as the zero position because then generally smaller bulges remain in the printing plate over the circumference of the printing cylinder, particularly at the beginning of the printing plate. These bulges can not be compensated even by subsequent tensioning at the rear end of the printing plate because the friction over the entire printing plate, which is already in contact with the printing cylinder, is too high to accomplish this. Modern printing plates are made of a material which must not be tensioned too much in order to prevent it from lengthening to an undue degree. The operator therefore sets the front tensioning bar back by a certain length, starting from the zero position defined by the front groove wall, and then tests it several times until the accurate zero position has been found by the correct setting of all tensioning screws of the front tensioning bar. It is obvious that this is not only labor intensive but also has the particular drawback that the result, in spite of the great labor effort, not always corresponds to the desired outcome, particularly since the zero position defined by the front groove wall had to be relinquished and the position of the front tensioning bar relative to the tensioning screws had to be changed several times, and this generally for all tensioning screws of the front tensioning bar. The noted result is that in the prior art the adjustment of the printing plate at the printing cylinder is not only labor intensive but, in spite of the great effort of labor, the desired zero adjustment is not always realized with sufficient reliability with the consequence of wasted material and renewed labor expenditures for further adjustments.